This invention relates to method and apparatus for sensing gas pressure in a container, and particularly to such method and apparatus for rapidly sensing whether the vacuum level in a closed container is above a predetermined minimum level, thereby to provide an indication of whether the container has, or has had, one or more gas leaks in its walls.
There are many applications in which it is desirable to sense the gas pressure within a closed container. For example, the container may contain a material which would be deleteriously affected by exposure to ambient air, as may occur if the container is not hermetically sealed but instead is subject to gas leakage through its walls. To provide indications of whether such a gas leak has occurred, the interior of the container may be provided with a partial vacuum when it is initially sealed, so that a later loss or reduction in this vacuum will indicate that leakage has occurred, with possible harm to material in the container.
One specific application of the invention, with respect to which it will be particularly described herein, arises in connection with food containers. It is well known that containers for many types of foods must be hermetically sealed since the food may otherwise be adversely affected by contaminating substances or organisms in the surrounding ambient air, or may undergo harmful chemical reactions when ambient air is permitted to leak into the container.
In filling food containers, it is common to leave a residual head space or empty portion at the top of the container, and to close and seal the container while its contents are still at an elevated temperature. Upon subsequent cooling of the container to room temperature, the pressure of the gas in the container will drop below the ambient level to produce a partial vacuum in the container. So long as the gas within the container remains at a substantially reduced pressure, i.e. at a predetermined minimum level of vacuum, it can normally be safely assumed that the sealing procedure provided a satisfactory gas seal, and that there has not been subsequent substantial gas leakage through the walls of the container. On the other hand, complete or partial loss of the vacuum in the container is properly taken as an indication that a gas leak has occurred, and that the food may have been deleteriously affected.
A variety of methods and apparatuses have been proposed in the prior art for the purpose of measuring or checking the presence of vacuum inside a closed container. One general class of such arrangements takes advantage of the fact that at least some portion of a wall of the container will move inwardly in response to a vacuum within the container, and will move outwardly toward its original position if the internal vacuum is lost. One class of such systems utilizes the continuous or progressive deflection of a "linear" diaphragm in proportion to the differential pressure between its opposite surfaces, the diaphragm being one of the usual walls of the container or a specially constructed portion of one wall of the container. In another class of such systems, a special snap-action diaphragm is provided in a portion of the walls of the container, which snap-action diaphragm has an inwardly-extending configuration when the gas pressure in the container is considerably lower than that of a surrounding ambient, an outwardly-extending configuration when the internal vacuum is absent or very low, and moves between these inward and outward positions with a very rapid snap action in response to gas pressure changes, rather than deflecting in a smooth continuous manner.
One difficulty encountered if one tries to use a linear or proportionally-deflecting diaphragm system to detect the level of vacuum in a container arises from the normal variations occurring in manufacture of the container, including the diaphragm walls, which produce differences between containers with regard to the absolute position of the diaphragm wall with respect to a reference portion of the container. Because of this, the deflected position of the diaphragm in different containers is not a reliable indication of the pressure within the container.
The snap-action diaphragm has a number of advantages over the proportional or linear diaphragm, and has been proposed for use in a number of different arrangements. Such a diaphragm acts somewhat like the bottom of the usual squirt-type oiling can, in that it maintains a stable relatively outward position when the differential pressure urging it inward is zero or of low value, suddenly snaps to a second, inwardly-deflected position when the differential pressure reaches a predetermined switching level, remains substantially in its second inwardly-deflected position for pressures above that switching level, and will revert to its original relatively outward position when the inwardly-acting differential pressure is reduced below a predetermined level. The differential pressure level at which the diaphragm switches to its inward position can be made relatively consistent as between different containers made by the same process. Such a diaphragm is therefore particularly useful in providing an indication of whether the differential pressure is above or below the level at which snap-action occurs, and for certain purposes this is all the information that is necessary, the exact value of the differential pressure not being required. With this approach, not only is it possible to produce containers having diaphragms with reproducible switching levels, but also the distinctive abrupt snap action is easy to detect and observe unambiguously.
U.S. Pat. Nos. 1,825,699 and 1,825,744 of Landrum issued Oct. 6, 1931 and Oct. 6, 1931 respectively, illustrate one manner in which snap-action diaphragms have been utilized previously. In U.S. Pat. No. 1,825,699, the snap-action diaphragm is normally and desirably deflected inwardly of the container by the vacuum in the container, and the container is placed within an evacuated chamber containing a vacuum sufficient to cause the diaphragm to snap outwardly only if the internal vacuum has fallen below a predetermined standard value; such outward flipping, if it occurs, indicates a faulty container, and is electrically sensed to operate a buzzer. In U.S. Pat. No. 1,825,744, the sound made by the diaphragm when it snaps outwardly is used as an indication of the faulty can. However, such a diaphragm which has suffered an undesired mechanical distortion due to impact or improper manufacturing may not snap outwardly in response to the applied test vacuum, even though the container may have lost its internal vacuum, and may therefore be classified as a satisfactory can when it is in fact faulty. U.S. Pat. No. 1,974,026 of Hicks discloses a somewhat similar arrangement in which a normally inwardly-deflected diaphragm is sucked outwardly by applied test vacuum of increasing magnitude until it encounters an electrical contact to sound a buzzer, the level of the test vacuum necessary to accomplish this being noted and taken as an indication of the internal vacuum of the container.
A common difficulty with such previous snap-action diaphragm systems for sensing internal vacuum lies in the fact that the containers and diaphragms are susceptible to types of damage which may change or eliminate the snap action in a manner which results in a test determination that the container has satisfactory vacuum when in fact it has not. Such distortions of the desired snap action can occur through loss of temper of the material of the container, through accidental mechanical deformation, or through excessive internal reduced pressure developing forces in excess of the yield point of the container material. Any of these conditions can cause the diaphragm to remain in its inwardly deflected position even though the vacuum has been lost. Under such conditions, there will appear to be a reasonably strong internal vacuum since the diaphragm remains in its inward configuration, and if an external vacuum is applied to pull it outwardly it may resist in a manner indicative of the presence of a substantial vacuum in the container.
Accordingly, it is an object of the invention to provide a new and useful method and apparatus for sensing gas pressure in a container.
Another object is to provide such method and apparatus which will reliably indicate whether the vacuum in the container is above or below a predetermined minimum acceptable level.
Another object is to provide such method and apparatus which will reliably indicate that the vacuum level within the container is above a predetermined minimum level only when such is actually the case.
A further object is to provide such an apparatus which will perform these functions rapidly, simply and reliably.
A further object is to provide such method and apparatus which does not require complex and expensive equipment for the purpose of making the vacuum measurements.